This invention relates to electrolytic processes using cells which incorporate particulate electrodes and, more particularly but not exclusively, is concerned with electrolytic processes involving the electrodeposition of metal onto the particles of a fluidised bed cathode.
In recent years there have been described particulate electrodes which comprise a number of particles at least the surfaces of which have an electrical conductivity of the order of 10.sup.2 .OMEGA..sup.-.sup.1 cm.sup.-.sup.1 or greater and which, when the particulate electrode is in use, are moving so as to be in intermittent contact, either directly or through the agency of intermediate particles, with at least one electrically-conductive member often called the "feeder electrode" by means of which electric current is conducted to the particles which engage in charge transfer reactions with species contained in the electrolyte. The electrical conductivity of the "feeder electrode" is generally of the order of 10.sup.4 .OMEGA..sup.-.sup.1 cm.sup.-.sup.1 or greater. These particulate electrodes have been developed in a number of different forms. In one form, a mixture of the particles of the electrode and an electrolyte is pumped through a portion of the cell which contains the feeder electrode and in which the electrode reaction occurs, around a circuit outside this portion and is then returned to the portion for further reaction. In another form, the particles of the electrode remain continuously within the portion of the cell which contains the feeder electrode for the entire length of time during which the particles are allowed to contribute to the electrode reaction, whilst the electrolyte is passed through this portion so as to intermix the particles of the electrode. Often included within this latter form are electrodes which, in operation, comprise an expanded bed of particles formed by an upward flow of electrolyte through the portion of the cell which contains the particles of the electrode. With sufficiently fast flow rates the particles become suspended in the electrolyte and the bed becomes expanded in volume usually by expansion in the vertical direction. In order to achieve uniformity of particle concentration in the horizontal plane it is necessary to produce a pattern of flow of electrolyte that is substantially constant through a horizontal cross-section within the bed of particles. The terminology of fluidised beds has been applied to this situation and many of the properties of fluidised beds are evident in the behaviour of these electrodes which are therefore frequently known as fluidised bed electrodes. The high surface area of a fluidised bed electrode makes possible either the efficient electrolysis of dilute solutions or the use of a high current per unit volume of cell and per unit volume of electrolyte; for example, in copper deposition, apparent current densities up to 3000 A/m.sup.2 and more have been used.
Examples of particulate electrodes including fluidised bed electrodes, and their use in various electrochemical processes are disclosed in, for example, British Pat. No. 1,194,181; U.S. Pat. No. 3,180,810; U.S. Pat. No. 3,527,617; U.S. Pat. No. 3,551,207; French Pat. No. 1,500,269 and Canadian Pat. No. 790,933.
In many applications of fluidised bed electrodes, the electrode reaction involves deposition of ions onto the particles or dissolution of the material of the particles. In these applications the dimensions of the particles change with time and there may be provision for removal and replenishment of particles. However, in processes involving the electrodeposition of metal onto the particles of a fluidised bed cathode, i.e., a fluidised bed electrode acting as a cathode, it has been found that when such processes are performed continuously over relatively long periods of time, i.e., under the usual conditions of commercial production of electrolytically-deposited metal, there is a tendency for the particles of the cathode to agglomerate and to adhere to the feeder electrode or to other parts of the cathode compartment. This tendency is detrimental to efficient cell operation on a commercial scale and, if allowed to proceed unimpeded substantially reduces the potentially high value of the use of fluidised bed cathodes in such applications as, for example, metal winning and metal refining.
We have now discovered a manner of performing processes in which there is electrodeposition of metal onto a fluidised bed cathode whilst ameliorating the abovementioned disadvantages.